HPLC determination of morphine with amperometric detection at low potentials under basic pH conditions

Summary A simple, sensitive, accurate and reliable method for morphine determination in biological samples has been developed. It uses reverse-phase HPLC on a polymeric column with an eluent (0.05 mol/L dibasic sodium phosphate: acetonitrile, 8515) at pH 9.5, allowing both the suppression of ionization of morphine amine and the promotion of oxidation of the phenolic group. Amperometric detection at mild oxidizing potential (350 mV) proves very selective, and, therefore, only a simple and rapid one-step liquid-liquid sample preparation is required. Under these conditions clean chromatograms are obtained even with complex biological matrices such as cadaveric blood, urine and hair.Minimum detectable amount of morphine is about 200 pg injected. A calibration line with a correlation coefficient of 0.99996 has been produced over the range 7.5–250 ng/mL. Precision results comparable to other HPLC methods.A preliminary report was presented at the 25th T.I.A.F.T. Meeting 1988, Groningen (NL), June 27–30 1988.     

Separation of pesticides by high-performance liquid chromatography with amperometric detection

The possibility of the amperometric detection of a number of pesticides, such as benomyl, thiram, linuron, metoxuron, desmedipham, dicuron, lenacil, and fludioxonil, widely used in agrochemical practice was studied. The effect of the working electrode material (glassy carbon, nickel, and gold) and the type of the electrochemical cell on the value of the analytical signal was studied using the example of thiram. It was found that the optimum potential of the working electrode in analyzing a pesticide mixture was 1400 mV. The dependence of the analytical signal on the pesticide concentration was shown to be linear. The detection limits for the analytes were calculated. Using a 100-μL sample loop, all of the studied pesticides can be determined at the level of the maximum permissible concentration (MPC). The amperometric determination of seven pesticides at the level of MPC in real samples was shown by the examples of model mixtures dissolved in tap water and beetroot juice.     

New automatized method with amperometric detection for the determination of azithromycin

A FIA-amperometric method for azithromycin determination was developed. A working glassy carbon electrode and a Ag/AgCl/NaCl (3 M) reference electrode were used. The determination is based on the electrochemical oxidation of the azithromycin at 0.9 V in Britton-Robinson buffer solution (pH 8.0). Due to the adsorption of the reaction products on the electrode surface, an effective cleaner cycle was implemented. By using the optimum chemical and FIA conditions, a concentration linear range of 1.0-10.0 mg L⁻¹ and a detection limit (LOD) of 0.76 mg L⁻¹ are obtained. The method was validated and satisfactorily applied to the determination of azithromycin in pharmaceutical formulations.     

Comparison of amperometric and UV-spectrophotometric monitoring in the HPLC analysis of pesticides

Summary Studies have been carried out to compare different detection techniques including amperometric detection in the reductive and oxidative mode and UV detection following the HPLC separation of pesticides. By the example of 4,6-dinitro-o-cresol (DNOC), 2-sec-butyl-4,6-dinitrophenol (Dinoseb) and 2-tert-butyl-4,6-dinitrophenol (Dinoterb), it is shown that the electrochemical detection exhibits higher sensitivities than the UV-technique. The detection limits are in case of the oxidative amperometric technique 0.1 ng for all pesticides, in the reductive mode 0.3 ng for DNOC and 1 ng for Dinoseb and Dinoterb. By UV-monitoring the detection limits were found to be 2 ng for DNOC and 24 ng for Dinoseb and Dinoterb.     

Determination of the pesticides fenthion and fenitrothion by flow injection with amperometric detection

The response of a glassy carbon amperometric detector in a flow system was studied. The values of the hydrodynamic variables were optimized. The detector functioned as a “thin-layer” cell. Methods are proposed for the determination of 1.6 × 10^?7?1.6 × 10^?5 Fenthion and 8 × 10^?6? 8× 10^?5 M Fenitrothion in a methanolic acetate-buffered carrier stream. The detection limits were 0.15 and 0.52 μM, respectively.     

Determination of clotiazepam by HPLC with spectrophotometric and amperometric detection

Summary Spectrophotometric and amperometric detectors were compared for the determination of clotiazepam by HPLC. Methanol-water (6040) containing 0.01 mol/l ammonium acetate and methanol-water (7030) containing 0.01 mol/l acetate buffer pH 4 were used as mobile phase in the spectrophotometric and amperometric detection, respectively. The minimum detectability for the spectrophotometric detection was 20 ng injected and the method was applied for the determination of clotiazepam in serum, whereas for the amperometric detection a limit of detection of 2.5 g injected was obtained, using this technique for clotizepam determination in tablets.

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Bestimmung von Clotiazepam durch HPLC mit spektralphotometrischer und amperometrischer Detektion

     

Pulsed amperometric detection of sulfur-containing pesticides in reversed-phase liquid chromatography

Pulsed amperometric detection at a gold electrode is demonstrated for the separation of eight sulphur-containing pesticides on a C-18 reversed-phase column with 50% (v/v) acetonitrile in acetate buffer (pH 5.0) as the mobile phase. Detection was based on a two-step potential waveform with adsorption of the analyte during cathodic polarization and subsequent amperometric detection catalyzed by oxide formation following anodic polarization. The mechanism of the anodic detection involves prior adsorption of the sulfur compounds. Hence, the shape of the calibration curve is strongly influenced by the adsorption isotherm of the analyte and, therefore, deviates from linearity at high concentrations. Detection limits below 100 ng ml^?1 in 20-μl samples (e.g., 0.8 ng of dimethoate) were obtained by using preconcentration from a larger sample onto a C-18 fore column prior to injection into the separation column.     

高效阴离子交换色谱-脉冲安培检测法定量测定低聚木糖样品中的低聚木糖

建立了低聚木糖样品中的木二糖至木六糖等低聚木糖的高效阴离子交换色谱定量测定方法,并根据低聚木糖的聚合度与色谱保留时间的线性关系,对木七糖和木八糖的保留时间进行预测。采用CarboPacTM PA200阴离子交换柱(3 mm×250 mm),以醋酸钠和氢氧化钠为淋洗液进行二元梯度洗脱,脉冲安培法进行检测。结果表明,木二糖至木六糖在0.804～8.607 mg/L质量浓度范围内的线性关系良好,检出限为0.064～0.111 mg/L,定量限为0.214～0.371 mg/L。将该方法用于低聚木糖产品的检测,3个添加水平的加标回收率为84.29%～118.19%,相对标准偏差(n=3)为0.44%～14.87%。结果表明该方法适用于低聚木糖产品中有效成分的快速、高效分离和定量测定。     

Analysis of tetracycline antibiotics using HPLC with pulsed amperometric detection.

The analysis of tetracycline, oxytetracyline, chlortetracycline and doxycycline by high-performance liquid chromatography with pulsed amperometric detection using an anodized boron-doped diamond thin film (BDD) electrode is originally reported. The analyses were carried out using the mobile phase, phosphate buffer (0.01 M, pH 2.5)-acetonitrile (80:20; v/v), on a C18 column (250 x 4.6 mm i.d., 5 microm) at a flow rate of 1.0 mL/min. The optimal PAD waveform parameters at the anodized BDD were 1.5 V (versus Ag/AgCl) detection potential (E(det)) for 290 ms (200 ms delay time and 90 ms integration time), 2.0 V (versus Ag/AgCl) oxidation potential (E(oxd)) for 200 ms oxidation time (t(oxd)) and 0.4 V (versus Ag/AgCI) reduction potential (E(red)) for 200 ms reduction time (t(red)). The proposed method showed the simultaneous determination of tetracycline, oxytetracyline, chlortetracycline and doxycycline with a linear range of 0.1 - 100 microg/mL, detection limits of 0.05 - 0.1 microg/mL and recoveries of 70.8 - 96.0%. The application of this method to real samples was demonstrated and validated using a shrimp sample.     

Supported liquid membranes for the determination of vanillin in food samples with amperometric detection

A supported liquid membrane system has been developed for the extraction of vanillin from food samples. A porous PTFE membrane is impregnated with an organic solvent, which forms a barrier between two aqueous phases. The analyte is extracted from a donor phase into the hydrophobic membrane and then back extracted into a second aqueous solution, the acceptor. The determination (100–1400 μg ml⁻¹ vanillin) was performed using a PVC-graphite composite electrode versus Ag/AgCl/3MKCl at +0.850 V placed in a wall-jet flow cell as amperometric detector. The solid sample is directly placed in the membrane unit without any treatment, and the analyte was extracted from the sample, passes through the membrane and conduced to the flow cell by the acceptor stream. The limit of detection (3σ) was 44 μg ml⁻¹. The method was applied to the determination of vanillin (9–606 μg g⁻¹) in food samples.     

Platinum particles-modified electrode for HPLC with pulsed amperometric detection of thiols in rat striatum

A new chemically modified electrode based on the immobilization of Pt particles is fabricated and exhibits electrocatalytic oxidation for L-cysteine (L-Cys), glutathione (GSH) and penicillamine (PEN) with relatively high sensitivity. It is also adaptable to HPLC for pulsed amperometric detection (PAD) of these thiols. PAD largely improves the detection sensitivity because the alternated polarizations can effectively clean and reactivate the electrode surface. It is shown that the peak currents of L-Cys, GSH and PEN are linear to their concentrations, with the calculated detection limit of 1.1 x 10(-7), 1.8 x 10(-7) and 3.8 x 10(-7) mol L(-1), respectively (S/N = 3). The method has been successfully applied to assess the contents of L-Cys and GSH in rat striatal microdialysates. The average contents of the two analytes in rat striatum are 2.6 x 10(-6) and 2.8 x 10(-6) mol L(-1), respectively.     

Optimization of a flow injection system with amperometric detection for arsenic determination.

A selective and sensitive analytical procedure for rapid arsenic determination by gas-diffusion flow injection analysis with amperometric detection was developed. The method is based on the arsenite reduction by NaBH(4). Derived arsine diffuses through a PTF membrane into the acceptor flow stream and is amperometrically determined on a platinum working electrode. The limit of detection (3 sigma) at room temperature was 5 microg/dm(3) of As(III). The relative standard deviation for a 1 mg/dm(3) As(III) standard was 1.96% for six repetitive injections. Arsenic(V) was determined after its prereduction with potassium iodide. Arsenic determination was not interferred with by 1 mg/dm(3) Sb(III), 5 mg/dm(3) Sn(II), 10 mg/dm(3) Se(IV), 1 mg/dm(3) As(V), 1 mg/dm(3) hydrasine, 1 mg/dm(3) Fe(II) or 0.5 mg/dm(3) Fe(III) solution. The throughput of this method was 60 analyses per hour. This method was successfully applied to arsenic determination in some power plant waste water samples.     

Electroanalytical study of sulphamerazine at a glassy-carbon electrode and its determination in pharmaceutical preparations by HPLC with amperometric detection

An electroanalytical study of the oxidation of sulphamerazine at a glassy-carbon electrode has been made by use of different voltammetric techniques. The limiting current is diffusion-controlled, at pH 7.0, for sulphamerazine in the concentration range 2.0 x 10(-5)-1.0 x 10(-4)M. The limits of determination found for differential pulse voltammetry at a stationary electrode and at a rotating electrode were 5.9 x 10(-6) and 3.3 x 10(-6)M respectively. A reversed-phase HPLC method with amperometric detection for the analysis of sulphamerazine, sulphadiazine and phthalylsulphathiazole mixtures has been developed, and applied to a commercial pharmaceutical preparation.     

A reversed-phase high-performance liquid chromatography method with pulsed amperometric detection for the determination of glycosides

We have developed a reversed-phase high-performance liquid chromatography pulsed amperometric detection (RP-HPLC-PAD) method for the determination of glycosides. It is sensitive, repeatable, and selective without the pretreatment step. Ginsenosides were separated completely in 50 min using an water-acetonitrile gradient as the eluent and detected by PAD under NaOH alkaline conditions. The ginsenoside detection limit (S/N=3) was 0.02-0.07 ng and the quantification limit (S/N=10) was 0.1-0.2 ng. The coefficient of linear regression was 0.9984-0.9998 for concentrations between 1 and 50 microg/mL. The intra- and inter-day precision (RSD) was less than 6.35% in Ginseng Radix and Shy-jiun-tzyy-tang extracts. The average recoveries from Ginseng Radix and Shy-jiun-tzyy-tang extracts were 98.19-105.45% and 96.89-102.22%, respectively.     

Sub-millimolar determination of formalin by pulsed amperometric detection

Formalin, formaldehyde in the presence of methanol, was determined by pulsed amperometric detection (PAD). A triple waveform using E_det=−0.3 V (t_det=30 ms), E_oxd=+0.8 V (t_oxd=200 ms), and E_red=−0.8 V (t_red=350 ms) versus Ag/AgCl was applied at a Au electrode for detection in a flow injection (FI) system. The approach was rapid and yielded a sub-millimolar detection limit (0.0129 mM) with a dynamic range up to 100 mM. A precision of 8.8% R.S.D. at 1.0 mM for two hundred repetitive injections by the FI-PAD was obtained, whereas holding at a constant potential (−0.3 V versus Ag/AgCl) for anodic oxidation of formaldehyde caused the response to decrease dramatically after a few measurements. The method developed was used to analyze the formalin contents of water from rinsed samples of vegetables and fruit and ice-melt from seafood, and the method showed good agreement with the liquid chromatography (LC) method.     

Separation of natural antioxidants using PDMS electrophoresis microchips coupled with amperometric detection and reverse polarity

This report describes the use of PDMS ME coupled with amperometric detection for rapid separation of ascorbic, gallic , ferulic, p‐coumaric acids using reverse polarity. ME devices were fabricated in PDMS by soft lithography and detection was accomplished using an integrated carbon fiber working electrode aligned in the end‐channel configuration. Separation and detection parameters were investigated and the best conditions were obtained using a run buffer consisting of 5 mM phosphate buffer (pH 6.9) and a detection voltage of 1.0 V versus Ag/AgCl reference electrode. All compounds were separated within 70 s using gated injection mode with baseline resolution and separation efficiencies between 1200 and 9000 plates. Calibration curves exhibited good linearity and the LODs achieved ranged from 1.7 to 9.7 μM. The precision for migration time and peak height provided maximum values of 4% for the intrachip studies. Lastly, the analytical method was successfully applied for the analysis of ascorbic and gallic acids in commercial beverage samples. The results achieved using ME coupled with amperometric detection were in good agreement with the values provided by the supplier. Based on the data reported here, the proposed method shows suitability to be applied for the routine analysis of beverage samples.     

Indirect determination of iron in a flow-injection system with amperometric detection

The change in peak current resulting from the reaction of Fe(II) with nitroso-R salt in a flow-injection system is used to quantify Fe(II) with either single- or dual-electrode amperometric detectors. The current change varies linearly with Fe(II) concentration from 0 to 200 mg 1^?1. The relative standard deviation was about 5% with the single-electrode detector and about 10% with the dual-electrode detector. The method is evaluated for the determination of iron in dietary supplements.